Sendmail is trying to relay your e-mail through an SMTP server located here: mail.example.com. This is the builtin default config for sendmail on this image.

What you want to do is to pass the google SMTP config for sendmail, like this:

echo "Subject: test" | sendmail -v -f your@mail.com -au<your-gmail-account> -ap<your-gmail-password> -H 'openssl s_client -quiet -tls1 -connect smtp.gmail.com:465' your@mail.com

A practical example for:

• gmail account: your@gmail.com
• gmail password: 123456

Would look like this:

echo "Subject: test" | sendmail -v -f your@mail.com -auyour@gmail.com -ap123456 -H 'openssl s_client -quiet -tls1 -connect smtp.gmail.com:465' your@mail.com

sendmail --help

For reference, here is the sendmail --help. You can get it from the image that you are using with:

docker run --rm php:fpm-alpine sendmail --help

BusyBox v1.29.3 (2019-01-24 07:45:07 UTC) multi-call binary.Usage: sendmail [-tv] [-f SENDER] [-amLOGIN 4<user_pass.txt | -auUSER -apPASS]                [-w SECS] [-H 'PROG ARGS' | -S HOST] [RECIPIENT_EMAIL]...Read email from stdin and send itStandard options:        -t              Read additional recipients from message body        -f SENDER       For use in MAIL FROM:<sender>. Can be empty string                        Default: -auUSER, or username of current UID        -o OPTIONS      Various options. -oi implied, others are ignored        -i              -oi synonym, implied and ignoredBusybox specific options:        -v              Verbose        -w SECS         Network timeout        -H 'PROG ARGS'  Run connection helper. Examples:                openssl s_client -quiet -tls1 -starttls smtp -connect smtp.gmail.com:25                openssl s_client -quiet -tls1 -connect smtp.gmail.com:465                        $SMTP_ANTISPAM_DELAY: seconds to wait after helper connect        -S HOST[:PORT]  Server (default $SMTPHOST or 127.0.0.1)        -amLOGIN        Log in using AUTH LOGIN (-amCRAM-MD5 not supported)        -auUSER         Username for AUTH        -apPASS         Password for AUTHIf no -a options are given, authentication is not done.If -amLOGIN is given but no -au/-ap, user/password is read from fd #4.Other options are silently ignored; -oi is implied.Use makemime to create emails with attachments.